1. Field of Invention
The present invention relates to a digital dimming device and method, in particular to one that uniformly distributes the illumination timings of multiple strings of light emitting devices.
2. Description of Related Art
In a circuit for controlling light emitting devices (such as light emitting diodes, LEDs), a dimming function is often required. Two dimming methods have been proposed in prior art to deal with the case where there are multiple strings of light emitting devices. The first one is shown in FIG. 1, wherein the same dimming control signal is used to adjust the brightness of every string of light emitting devices. The second one is shown in FIG. 2, wherein different dimming control circuits are used to dim corresponding light emitting device strings. More specifically, in the first prior art shown in FIG. 1, a light emitting device control circuit comprises a power stage control circuit 21 for controlling a power transistor in a power stage 22 to convert an input voltage Vin to an output voltage, which is provided to multiple LED channels CH1-CHn. The power stage circuit 22 may be, for example but not limited to, asynchronously or asynchronously buck, boost, buck-boost, inverting or fly back voltage converter shown in FIGS. 3A-3G, wherein if the power stage circuit 22 is the fly back voltage converter shown in FIG. 3G, the power stage control circuit 21 and the dimming control circuit 23 are usually separated in different integrated circuits. In other cases, the power stage control circuit 21 and the dimming control circuit 23 can be integrated in the same integrated circuit. The dimming control circuit 23 provides the same dimming control signal to all LED channels CH1-CHn, synchronously controlling the transistors Q1-Qn on the channel paths. An example of such prior art is U.S. Pat. No. 7,259,687.
In the second prior art shown in FIG. 2, different dimming control circuits 23A, 23B, and 23N are used to adjust the brightness of corresponding light emitting device strings. An example of such prior art is US Patent Publication No. 2009/0134817.
However, if the same dimming control signal is used to synchronously control all light emitting device strings, all light emitting devices would be synchronous in their ON/OFF cycles, which would cause a larger ripple in the output voltage and current, and also a more serious flicker effect. A better arrangement is to turn ON the light emitting device strings in sequential order and to uniformly distribute the illumination timings of the light emitting device strings. Although the illumination timing of each light emitting device string can be independent from another string by controlling the light emitting device strings respectively, this can not ensure that the illumination timings of the light emitting device strings are uniformly distributed.
Besides, the frequency of a digital dimming signal is in a range of about 60-500 Hz, but in some applications it is difficult to provide a signal of such low frequency.
In view of the above, the present invention proposes a digital dimming device and method which can solve the problem of non-uniform distribution of illumination timings, and furthermore it can receive digital dimming signals in any frequency range.